Melanism in a larval Lepidopteran : repeatability and heritability of a dynamic trait.

1. Although it is well established that the deposition of melanin pigment in the cuticle of larval Lepidoptera is influenced by both environmental and genetic factors, few studies have examined intra-individual regional variation in the degree of melanism or the ontogenetic dynamics of this trait. Here, heritable and density-dependent effects on within-individual and stage-specific variation in melanism were examined in caterpillars of the Egyptian cotton leafworm, Spodoptera littoralis (Boisduval). 2. Using quantitative spectrometric methods, it is shown that cuticular melanism changes dramatically within larval stadia, showing the highest and lowest levels of melanism early (first day) and late (final day) in each larval stadium respectively. However, solitary-reared caterpillars were significantly paler than those reared gregariously at all stages of development and maintained greater levels of variation in melanism. This variation in melanism was repeatable and exhibited a significant heritable component (narrow sense heritability based on offspring–parent regressions: h2 = 0.18–0.30). 3. The degree of melanism was correlated negatively with larval body weight in solitary caterpillars, but not gregarious ones. Melanism also varied spatially, with the lateral longitudinal band being consistently darker than the dorsal or dorso-lateral bands. Crowd-rearing increased melanism in all regions of larval cuticle, but the extent of crowding-induced melanism was more pronounced in the dorsal and dorso-lateral bands than in the lateral one. 4. These results indicate that although cuticular melanism is a highly dynamic trait, ontogenetic changes in relative cuticular melanism are both predictable and repeatable within individuals and genotypes. This has implications for our understanding of the evolution of melanism and for applying artificial selection on the basis of colour

Group living and investment in immune defence : an inter-specific analysis.

1. Since parasite transmission is often density-dependent, group living is normally thought to lead to an increased exposure to parasitism. As a consequence, it is predicted that animals living in groups will invest more resources (energy, time, risk, etc.) in parasite defence than those living solitarily. 2. We tested this prediction by measuring basal immune parameters in the larvae of 12 species of Lepidoptera, grouped into six phylogenetically matched species-pairs, each comprising one solitary feeding and one gregariously feeding species. 3. Contrary to expectation, the solitary species in all six species-pairs had higher total haemocyte counts than the gregarious species, and in five out of six species-pairs the solitary species also exhibited higher phenoloxidase activity. Both measurements were positively correlated with each other and with the magnitude of the cellular encapsulation response. 4. The relationship between infection risk and group living was investigated with a dynamic, spatially explicit, host–pathogen model. This shows that when individuals aggregate in groups, the per capita risk of infection can be reduced if the lower between-group transmission more than compensates for the higher within-group transmission. 5. We conclude that the expectation that group living always leads to increased exposure to pathogens and parasites is overly simplistic, and that the specific details of the social system in question will determine if there is increased or decreased exposure to infection